It's dependent on speech recognition support. Go to about:config and try changing media.webspeech.recognition.enable and media.webspeech.recognition.force_enable to true.

You can use Windows' speech to text by pressing Win+H.

But in YouTube it's working

the proposed fix is not working. Please don't make us go to Chrome to use voice search!!

Google shows a "Search by voice" microphone icon for me with default settings.

Is it still missing in Troubleshoot Mode?